Some conventional engines (internal combustion engines) have two parallel modes of cylinder injection (direct injection (DI)) and port injection. The engines achieve homogeneous combustion in the cylinder having a uniform concentration distribution of air-fuel mixture therein, and stratified combustion in the cylinder containing a dense air-fuel mixture concentrated around a spark plug in a layered form.
The typical fuel injection control mainly involves the port injection during the homogeneous combustion, and the direct injection during the stratified combustion. The selection of an appropriate fuel injection mode depending on the operating state of an engine and the load thereon can improve the fuel economy while maintaining the output of the engine and combustion stability.
In some of the engines, the period of opening of the intake valve overlaps with that of the exhaust valve to improve the efficiency of scavenging the exhaust air from the cylinder and the efficiency of air intake. In particular, an engine including a turbocharger in its intake and exhaust system can improve in the charging efficiency and engine output by increasing the valve overlap (overlap period). This prompts the development of techniques to vary the timings of opening and closure of the intake and exhaust valves of the engine, to control the valve overlap in response to parameters such as the engine load.
The increased valve overlap, however, causes blow-by more often, which means that the intake air introduced from the intake path directly flows into the exhaust path. Then, the engine in the port injection mode, which injects the fuel into the intake system, makes a flow of the fuel passing through the cylinder to the exhaust path. This phenomenon may impair the engine output and environmental performance.
To solve the above problem, techniques have been disclosed for preventing the blow-by of fuel by regulating the volume of fuel of the port injection. For example, Patent Literature 1 discloses a control to determine whether the current operating state causes blow-by of fuel and inhibit the port injection in response to the results of determination. The determination depends on the intake pressure, the exhaust pressure, and the number of revolutions of the engine (the engine speed). The control based on such determination can regulate blow-by of fuel caused by the valve overlap.